small Bahtinov 2

Yesterday, I had made my little Bahtinov mask for the camera lens and was unsatisfied with the results. I wanted to try again. Still not able to find any specific recommendations for overall size, slot thickness, structural bar thickness, once again I picked my own values.

Let's try bigger. What the heck. It's prototyping! I made a mask with an 80mm opening, 4 mm slots and bars. All bars. The blocking bars and the structural bars I made 4mm wide. And took a cue from a finished model by Steve Richards: the centre "top" bar would be a large triangle.

Fired up the false star. Put the camera on the tripod. Zoomed in, mechanically, and on the view screen. Another strange bumpy blob. No nice long diffraction spikes. Damn it. Larger was not working. Back to Google...

Stumbled across a design in one of the forums, again, that I had seen before, one produced on a 3D printer. It had very fine slots. Dozens of them in the small cap that fit a camera lens. 1mm wide! Wow. The designer said it was pretty cheap for him. I immediately put out some queries for a 3D printing shop in the GTA.

I considered the Hartmann style mask, with the two circles, or variants, for example with the triangles. But I didn't like the idea of loosing so much light.

And then, in another forum thread, back at the Stargazers Lounge, at last I found one missing piece of information: the clear aperture for a camera lens. In fact, they gave the formula.

clear aperture = focal length / largest f-stop

I ran the numbers for the 18-55 lens. Crikey! 3 to 4mm. Now I understood why I was seeing a blobby lumpy things opposed to nice diffraction spikes. The thick structural bars I was using, 5mm, even the 4mm, were covering most of the aperture! I'd definitely need to go small, fine, delicate.

Sheesh, how would you do this at home? Somebody suggested using thread.

This person also talked about a "Y-mask" they used. Showed a photo. Huh. Different.

Spotted a design where the person had used a men's small hair comb. Ha! I had actually thought of that briefly yesterday... No. Really! 'Course, I haven't used a comb for years! But a trip to the Dollar Store might yield the parts needed for cheap. Clever.

Read some interesting remarks about the Lord mask. Noted the reference to Chris Lord's Primer on Fraunhofer Diffraction. Details of the "first order" and "third order" diffraction lines. From someone who sounded like they really knew what they were talking about.

The takeaway? A simple Y-mask, for small apertures, was as good or better than a Bahtinov mask. With the added benefit that it passed a lot more light, possible a full f-stop. Which would certainly be handy for those faint stars...

So, maybe, my whole approach to this point was wrong...